Experimental investigations of aerodynamic sound generation due to vortex-airfoil interaction in transonic flow were performed. For that purpose two different experimental set-up have been applied: A vacuum operated transonic wind tunnel and a newly built shock tube. Flow visualization was accomplished by a Mach-Zehnder interferometer as well as by a holographic interferometer. The interferograms obtained were evaluated by an image processing system developed at the institute. Two mechanisms of aerodynamic sound generation could be identified which are basically different from the dipole-like one which has been known from subsonic flow fields. The newly found sound waves are called 'transonic waves' and 'compressibility waves'. For both of them simple models have been suggested which can explain their occurrence and -- at least partly -- also their strengths. Beside the sound generation, viscosity effects were investigated, such as flow separation at the leading edge, secondary vortices at the shoulder of the airfoil, and the Kutta-condition at the trailing edge. But in transonic flow their influence on the sound generation seems to be of secondary importance only.